What amp circuit required for 2 ovens and a hob

[SUNRAY]

From what im rrading this seems to be a bone of contention amoungst electricians even though the regs seem to support it.

For i just want to be sure that if i turn on teo ovens at 200c and have two maybe 3 things on the go on the hob at the same time it wont trip. This for me would be worst case "Xmas Day" scenario.

As it stands oir electrician is saying no possible which means we would have to move where we want the ovens on the island under the hob to another location in the kitchen which quite frankly looks naff.

Sorry I missed this earlier, although it does cause issues occasionally in the domestic situation, correctly applied diversity is usually not a problem. Here at home we have an 11.2KW ceramic top cooker, when I installed it I had to extend the feed and to get the job done in a hurry I put a piece of 2.5mm² T&E in temporarily and as I had a 25A MCB handy I used it, diversity comes out at ~22A. A couple of days later I replaced the temporary T&E. Some 10 or so years later it tripped when every element was in use, by then I'd made a 32A MCB spare and changed it over quickly, the only other time it tripped was a faulty element after I'd changed to RCBO and that was I suspect on RCD action.

I imagine running your appliances on 32A will be fine unless you do mega catering, If your cable is 6mm² it might be acceptable to upgrade to 40A (your electrician should be able to assess your installation) and that should cater for a 25KW set-up.

The thing that should be done is follow instructions regards to 16A MCB for the ovens as the wiring will be too small for 32A MCB.

 

[JohnW2]

I'm glad I'm not going mad. I didn't do any calculations, just worked on gut feeling... ... But it has to be done.
Diversity of 7KW @ 230V = 16.13A, @ 240V = 15.75A

I think that the ('abbreviated') way you have written that might confuse some readers - so, to clarify (for them!)...

....if, as is usually the case,the 7kW spec relates to 240V (hence about 29.17A at 240V) then, at that 240V, the after diversity current would (as you say) be about 15.75A [ 10 + {0.3 x (29.17 - 10)} ].

However, at 230V (the voltage we use for design calculations), the current drawn by such a ( "7kW at 240V" ) load would only be about 27.95A (hence a power of about 6.429 kW) and the after-diversity current would be about 15.385A [ 10 + {(0.3 x (27.95 -10)}]

20A of diversity @ 230V = 9.966KW, @240v = 10.4KW

Indeed - and, perhaps more dramatically, and relevant (since cooker circuits are commonly 32A), to get 32A 'after diversity would require a 'before diversity' current of 83.3A - 18.17 kW at 230V or 20.0 kW at 240V !

Well I'm gobsmacked, my gut let me down, I didn't expect those powers to be that high and certainly never thought It should be 6 or 10mm² to run an appliance on a 20A OCPD or that it was likely to only run for a minute if all switched on at the same time.

I'm not too sure what you're saying here. Both the OPD and cable can be sized according to the after-diversity current and, on that basis, 4mm² cable will often be adequate and 10mm² very rarely required.

Kind Regards, John

 

[JohnW2]

Sorry I missed this earlier, although it does cause issues occasionally in the domestic situation, correctly applied diversity is usually not a problem.

Indeed. The concept of diversity is probabilistic (dealing with 'averages' of relatively random events) and, as such, will inevitably occasionally 'fail', but only very rarely if the diversity calculation is reasonably 'conservative' ('cautious') - and the worst that happens if it does 'fail' is that an OPD will trip/'blow'.

Kind Regards, John

 

[ebee]

Well my take on it is :-
Although we all redeclared our nominal voltages some years ago I think all countries retained their long loved voltage ranges in practice.
So the UK is still running pretty much on 240v or thereabouts as we "always did".
Manufacturers "should really" be quoting at 230v nominal but there must surely be a big temptation for them to quote the elements if run on 240v therefore looking a bit more powerfull?
If one manufacturer does that then the others will see that and the added temptation is to do likewise for parity.

Regarding Diversity.
In my opinion it should be applied to the set of cooking appliances supplied from one circuit just like the age old tradition cookers would have been. All other considerations being equal it makes no difference if it`s not one appliance but split into 2, 3 or seven million appliances, it is the total possible load we are concerned with, to that figure we apply diversity providing we feel that diversity is appropriate.
I would not apply diversity to each appliance seperately then add them together.

 

[bernardgreen]

We are assuming that the appliance is following the rules of diversity, it may be that the appliance can run for long periods of time pulling the full rated load.

 

[Murdochcat]

If the OP is bothered by this just substitute the chosen induction hob for a model that plugs into a 13A socket and all will be OK.

As it stands the worse that could happen is the MCB or RCBO might trip - which I doubt would happen

 

[ebee]

We are assuming that the appliance is following the rules of diversity, it may be that the appliance can run for long periods of time pulling the full rated load.

I don`t think we can claim that there are "rules of diversity" to follow as such Benard.
Well OK rules as in "Rules of Thumb" I suppose.
The problem with rules of thumb is that they are just that, they often work but they are a guidance in effect and not a "law" of things.
That why I mentioned diversity if we feel it is appropriate.
I think it would be in most cases, moreover within normal domestic settings.
In an enterprise cooking 24/7 you would really need to adjust that quite a bit I think but in a bog standard family of 2 adults and 2.4 children or thereabouts our normal diversity rules seem to work pretty much OK, even on Xmas day, which could be one of the more testing/taxing parts of the year in most domestic situations.
I don`t see a need to alter our tried and tested usual diversity method - unless someone else has evidence of course.

 

[EFLImpudence]

We are assuming that the appliance is following the rules of diversity,

Yes, domestic cooking appliances. No assuming.

it may be that the appliance can run for long periods of time pulling the full rated load.

Then we shall not apply diversity.

What had you in mind? Something with multiple showers or immersions?

 

[SUNRAY]

I think that the ('abbreviated') way you have written that might confuse some readers - so, to clarify (for them!)...

....if, as is usually the case,the 7kW spec relates to 240V (hence about 29.17A at 240V) then, at that 240V, the after diversity current would (as you say) be about 15.75A [ 10 + {0.3 x (29.17 - 10)} ].

However, at 230V (the voltage we use for design calculations), the current drawn by such a ( "7kW at 240V" ) load would only be about 27.95A (hence a power of about 6.429 kW) and the after-diversity current would be about 15.385A [ 10 + {(0.3 x (27.95 -10)}]

Indeed - and, perhaps more dramatically, and relevant (since cooker circuits are commonly 32A), to get 32A 'after diversity would require a 'before diversity' current of 83.3A - 18.17 kW at 230V or 20.0 kW at 240V !

I'm not too sure what you're saying here. Both the OPD and cable can be sized according to the after-diversity current and, on that basis, 4mm² cable will often be adequate and 10mm² very rarely required.

Kind Regards, John

Going back to my cooker which ran for a lot of years on a 25A MCB, theoretically a 2.5mm² cable could be totally suitable for the circuit. However being >11KW it would not be unreasonable to imagine it pulling 40A for substantial periods of time while preparing the Christmas feast and the MCB may never trip at 40A (I haven't looked this up).

 

[JohnW2]

So the UK is still running pretty much on 240v or thereabouts as we "always did".

Indeed.

Manufacturers "should really" be quoting at 230v nominal but there must surely be a big temptation for them to quote the elements if run on 240v therefore looking a bit more powerfull?

Quite so.

Regarding Diversity. .... In my opinion it should be applied to the set of cooking appliances supplied from one circuit just like the age old tradition cookers would have been. All other considerations being equal it makes no difference if it`s not one appliance but split into 2, 3 or seven million appliances, it is the total possible load we are concerned with, to that figure we apply diversity providing we feel that diversity is appropriate.

All agreed.

I would not apply diversity to each appliance seperately then add them together.

Again agreed - but I'm not sure that anyone has suggested otherwise, have they?

Kind Regards, John

 

[JohnW2]

Going back to my cooker which ran for a lot of years on a 25A MCB, theoretically a 2.5mm² cable could be totally suitable for the circuit. However being >11KW it would not be unreasonable to imagine it pulling 40A for substantial periods of time while preparing the Christmas feast and the MCB may never trip at 40A (I haven't looked this up).

Provided only that the CCC of the cable is not less than the In of the OPD, that OPD is deemed adequate to protect the cable from a high enough current for a long enough period of time to do any harm to the cable.

Kind Regards, John

 

[JohnW2]

We are assuming that the appliance is following the rules of diversity, it may be that the appliance can run for long periods of time pulling the full rated load.

It's really the other way around - the "rules of diversity" we use are a (fairly 'conservative') estimate of how cooking appliances behave in the real world.

The 'without diversity' situation with cooking appliances would clearly not correspond with thee real world - if a hob really did (could) use 7kW continuously for long periods,without any reduction in that due to thermostatic control, all sorts of things would probably 'melt' and it would cost a fortune to run!

Ignoring thermal 'losses' (to the environment), in the real world the average (i.e.'after diversity') energy consumption will be determined (and 'limited') by the amount of heat transferred into the cooking utensils and their contents.

Kind Regards, John

 

[JohnW2]

I don`t think we can claim that there are "rules of diversity" to follow as such Benard. Well OK rules as in "Rules of Thumb" I suppose.

As I'vee just written to bernard, I think that the "rules" ('of thumb') attempt to relect the real-world behaviour of cooking apliances,rather than the converse.

I think it would be in most cases, moreover within normal domestic settings. .... In an enterprise cooking 24/7 you would really need to adjust that quite a bit I think but in a bog standard family of 2 adults and 2.4 children or thereabouts our normal diversity rules seem to work pretty much OK, even on Xmas day, which could be one of the more testing/taxing parts of the year in most domestic situations.

Although even "the rules" reflect that view,I'm not convinced that there is all that much difference between domestic and commercial situations, since the diversity we deal with only really relates to relatively short periods of time (e.g. up to about 30 mins).

Whether in a domestic or commercial kitchen, the 'worst case'we have to consider is that in which all the 'rings' of a hob simultaneously start heating up pans with 'cold' contents. Over a significant (e.g. 30 min) period of time, thermostatic control of the heating elements will result in a fairly dramatic reduction in average energy consumption - and that's what our ';after-diversity' figures represent.

As I see it,it doesn't make a lot of difference as to whether that 30-minute period happens once or twice per day (as in domestic kitchen) or is repeated by multiple consecutive 30-min periods for 18 hours per day - in either situation the 'worst case' average ('after-diversity') consumption should be much the same, shouldn't it?

I don`t see a need to alter our tried and tested usual diversity method - unless someone else has evidence of course.

Quite - as will all 'non-broken' things, they don't need 'mending'

Kind Regards, John

 

[ebee]

However I was taught it applies to individual appliances

3800/230 = 16.52A 10+(6.52*0.3) = 11.95A

7350/230 = 31.96A 10+(21.96*0.3) = 16.59A

16.59+11.95+11.95 = 40.5A

John,
perhaps I`ve missunderstood that one

 

[JohnW2]

However I was taught it applies to individual appliances
3800/230 = 16.52A 10+(6.52*0.3) = 11.95A
7350/230 = 31.96A 10+(21.96*0.3) = 16.59A
16.59+11.95+11.95 = 40.5A

John, perhaps I`ve missunderstood that one

No, you've not misunderstood anything, but I had completely missed and/or forgotten it !

People are obviously free to apply diversity separately to each appliance, and then add up the after-diversity currents, if they wish to - but such an approach will be much less 'generous' (in terms of diversity) than if one did it for the total cooking appliance load on the circuit - since if one does it separately for each appliance,one's after-diversity figures includes a "10A minimum" for each and every appliance (whereas there is only one "10A" if all appliances are considered together).

As an example,with Sunrays figures above, if both loads are on the same circuit, then applying diversity to the two together would result in an after-diversity figure of about 21.5A, not much more than half Sunray's 40.5A figures when the appliances are considered separately.

Kind Regards, John